Technical Field
This disclosure relates to geolocation with radio-frequency ranging.
Description of Related Art
Radio-frequency (RF) ranging technology provides distance and relative position between objects without the need to take mechanical measurements. RF ranging systems calculate the distance between two objects based in part on the time a radio signal propagates between those objects. In air, radio signals propagate at a constant rate, roughly equal to the speed of light.
Knowing the absolute position of a sufficient number of objects (such as e.g., cell phone towers fixed relative to the earth), an RF ranging system can be used to determine the absolute position of other objects. An RF ranging system may utilize a signal from the Global Positioning System (GPS) to provide absolute location. In many situations, however, GPS signals are either unavailable or actively denied to a potential user. RF ranging systems can provide accurate information regarding distance and/or location even in locations or situations without access to GPS signals. Some positioning systems, for example, GPS-based systems measure a time-difference of arrival (TDOA) of several synchronized signals to aid in position calculations.
RF ranging systems may utilize a round-trip time-of-flight (“RTTOF”) measurement to compute the distance between two radios. These types of systems can be further classified into “round-trip full-duplex” configurations and “round-trip half-duplex” configurations. In a round-trip full-duplex configuration, a first radio transmits a signal to a second radio, which then retransmits the same signal back to the first radio without performing any calculations using the signal. After receiving the retransmitted signal from the second radio, the first radio compares the departure time to the arrival time to calculate the round-trip signal propagation time. The system multiplies this time by the speed of light and divides by two to estimate the distance between the two radios.
In a round-trip half-duplex configuration, a first radio transmits a signal to a second radio, which then performs calculations using that signal. The second radio then transmits a new signal, which often contains the results of the calculations performed by the second radio, back to the first radio. The first radio then utilizes the data from the second radio and other data within the first radio to calculate the round-trip signal propagation time. The system multiplies this time by the speed of light and divides by two to estimate the distance between the two radios.
An improved RF-ranging technique is disclosed in U.S. Pat. Nos. 8,199,047 and 8,314,731 (which are assigned to the same assignee as the present patent application), the disclosures of which are hereby incorporated by reference in their entirety.
In many position determining systems, it is difficult to determine and track the positions of many objects simultaneously. This is often because several RTTOF measurements must be coordinated with each object being tracked and because systems often use synchronized clocks, which can be expensive to implement.
As used herein, a “radio” refers to a device that radiates, emits, transmits and/or receives electromagnetic signals (such as RF signals). Such devices may be generally referred to as “emitters,” “transmitters,” receivers,” or “transceivers” in the art and may radiate, emit, transmit, and/or receive electromagnetic signals. Radios may also include multiple-input, multiple-output (MIMO) devices. Such devices may be metallic or non-metallic and may include one or more radiating elements, emitting elements, transmitting elements, receiving elements, and/or transceiving elements, depending on the type of signal being radiated, emitted, transmitted or received.
When determining the position of an object, calculations may be performed by a data processor. The calculations may inherently produce finite errors in the real-time estimates of the position, velocity, and attitude, collectively known as “navigation errors.” If uncorrected, these errors grow unbounded with time. To help bound the navigation errors, it is common in the art to employ various filtering techniques. One class of filters is known as Kalman filters. The term “Kalman filter” will be used to collectively refer to members and variants in this class of filters, including but not limited to extended Kalman filters (EKF) and unscented Kalman filters (UKF). Another class of filters includes particle filters.
There remains a need for an improved geolocation system and/or method to allow more accurate computation of the current or real-time position of objects.